Among the nodes of a mobile communication network (UTRAN: Universal Mobile Telecommunication Terrestrial Radio Access Network) specified by the 3GPP (3rd Generation Partnership Project), a user equipment (UE), which is also called as a mobile station, receives packet data from a gateway (GW) through a base station.
A handover (HO) between base stations is performed when the user equipment (UE) moves the area position thereof from an accommodating base station (movement-source base station, written as Source evolved Node B: S-eNB) presently receiving packet data to a neighboring base station (movement-target base station, written as Target evolved Node B: T-eNB). Here, as to the reception of downlink packet data from the gateway (GW), the continuity of the received data is desired for the user equipment (UE).
FIG. 1 is a diagram illustrating an exemplary configuration of handover (HO) between base stations in the UTRAN of the mobile communication system.
There is assumed a case that a user equipment (UE) receiving data from a gateway (GW) through a source base station (S-eNB) is presently located in the area of a target base station (T-eNB), accompanying a handover from a source base station (S-eNB).
In such the case, in regard to a packet data in the gateway (GW), the user equipment (UE) is to receive the data through the target base station (T-eNB) as a data transferred from the source base station (S-eNB) to the target base station (T-eNB) and a data transmitted directly from the gateway (GW) to the target base station (T-eNB).
Here, according to the configuration illustrated in FIG. 1, the user equipment (UE) can form reception data in correct order (i.e. sequential), through packet data order arrangement using a packet data convergence protocol (PDCP) between with the gateway (GW).
More specifically, using sequence numbers in the PDCP between the gateway (GW) of a data transmission source and the receiving user equipment (UE), PDCP reordering (sequence reconfiguration) processing is performed in the user equipment (UE). By this, it is possible to guarantee the order of the received packets, even if the packet order is reversed at the time of the output from the gateway (GW) or caused by the base station (eNB) and the IP network located on the half way.
In contrast, according to the configuration of 3GPP LTE (Long Term Evolution) proposed by the standardization project (3GPP: the 3rd Generation Partnership Project) for the third generation mobile communication system, UTRAN in the mobile communication system illustrated in FIG. 1 becomes as illustrated in FIG. 2.
Namely, the PDCP function having been installed in the gateway (GW) is moved to a base station (eNB). As a result, PDCP is to be exchanged between the base station (eNB) and the user equipment (UE). In this case, there is no protocol for guaranteeing packet order end-to-end between the gateway (GW) and the user equipment (UE).
Therefore, it is no more possible to guarantee the order of the received packets between the gateway (GW) and the user equipment (UE).
To describe in more detail, in the IP network, any IP packet having a predetermined maximum transfer unit size (hereafter referred to as MTU: Maximum Transfer Unit) or larger is divided into packets by the MTU length.
FIG. 3 is a diagram illustrating an example of packet transfer in the above IP network. In FIG. 3, no mechanism is provided to distinguish the order in case of IP packets each having the MTU length or smaller. As a result, packets may possibly arrive after the packet transfer order is reversed due to a large delay through a changed route etc.
In the above case, although packets “0-1” and “0-2” divided by the MTU size can be assembled on the receiving side, it is not possible to assemble undivided packets 1, 2 of which order is reversed.
Therefore, in the IP network transfer, it is not possible to restore to a correct order when the order has been reversed due to the route change etc.
As a result, in the configuration of 3GPP LTE (Long Term Evolution) as illustrated in FIG. 2, a reverse data order may possibly occur in the cases of both data transfer from the source base station (S-eNB) to the target base station (T-eNB) and direct transmission from the gateway (GW) to the target base station (T-eNB) due to the route change. In the above cases, the continuity of data reception may not be obtained in the user equipment (UE).
For example, in the documents 1, 2, descriptions are given on the mobile communication network nodes (UTRAN: Universal Mobile Telecommunication Terrestrial Radio Access Network) specified by the 3GPP.    [Document 1] TS36.300 v 8.3.0 (Chapter 19)    [Document 2] TS36.413 v 2.0.0 (Chapters 8.2.1, 9.1 and 9.2)